Keep your masks away from UV light 
(sunlight or fluorescent bulbs). Keep them in their natural shape by stuffing them with plastic bags from the grocery store. You can use a wig head form, but I use stands that I make from various found objects at hobby stores or home improvement stores. Be creative. For a generic stand start with a dowel rod and a piece of 1x 6 scrap wood.

Customizing your mask
If you have decided to order a piece with vision/breathing holes and it fits a little too large you can crazy-glue a piece of sponge or foam rubber in the top, back, or both, so that your eyes match up with the vision slits better. If you would like to enlarge the vision slits, try the mask on to see which side the slits need to be bigger. Remove the mask and trim the holes slightly larger with scissors or an exacto knife. NEVER trim a mask while wearing it. If your mask is too tight, you can make the slit in the back of the mask larger.